This invention relates to a converging light transmitting glass body whose refractive index gradually changes in a direction transverse to a light path, and to a process for production thereof.
It has already been elucidated that a converging light transmitting glass body whose refractive index gradually changes from its center toward its surface in a direction transverse to a light path, and which has an ideal refractive index distribution showing an ideal change in refractive index from its center to its surface, namely an ideal refractive index that imparts a distortion-free lens action from its center to its surface has the following relation ##EQU1## wherein A is a refractive index distribution constant, and in an arbitrary phantom cross section of the light transmitting body taken at right angles to the light path, N.sub.o is the refractive index at the central point of the cross section, and N(r) is the refractive index at a distance r from the central point, with regard to meridional rays, and the following relation EQU N(r)=N.sub.o (1+A.r.sup.2).sup.-1/2 ( 2)
wherein all symbols are as defined, with regard to helical rays [see a Japanese-language publication, "Optical Technology Contact", vol. 16, No. 5, pages 25-37, an article written by Kooichi Nishizawa and entitled "Selfoc Lens and Application Thereof"].
If the equations (1) and (2) are polynomially expanded, they can be written as follows: ##EQU2## (with regard to meridional rays). ##EQU3## (with regard to helical rays).
A converging light transmitting glass body has previously been known which has a refractive index gradually changing from its center to its surface (see U.S. Pat. No. 3,941,474). This light transmitting body has the following relation ##EQU4## wherein in a phantom cross section taken at right angles to the path of light, N.sub.o is the refractive index at the central point of the cross section, and N(r) is the refractive index at a distance r from the central point.
A comparison of equation (3) with equations (1)' and (2)' shows that this light transmitting body has a refractive index approximate to a light transmitting body having an ideal refractive index distribution, but it still never has the ideal refractive index. Specifically, the above known light transmitting body shows refractive indices well corresponding with a light transmitting body having an ideal refractive index distribution in areas located at a small distance r from the central portion of the light transmitting body, but as the distance r from the central point increases (i.e., nearer to the peripheral surface portion), the known light transmitting body shows a smaller refractive index than the refractive index of the light transmitting body having the ideal refractive index distribution.
Because of having the aforesaid refractive index distribution, the conventional light transmitting body has the following two points to be improved for practical application.
Firstly, it has the defect that since it shows a refractive index remote from the refractive index of the light transmitting body having an ideal refractive index distribution as it departs from the central portion, transmission of light information becomes obscure and lacks accuracy. In order to remedy this defect, it is the practice to utilize the central portion of the light transmitting body which has a refractive index distribution close to the ideal refractive index distribution. Applications in which the light transmitting body can be used in this way are naturally limited, and the efficiency of utilization is reduced as a matter of course.
Because of the aforesaid refractive index distribution, conventional light transmitting bodies will not fully withstand use in applications which require high performance and have been actively developed in recent years, such as optical detecting devices for videodiscs, optical image viewing devices (e.g., an arthroscope), and various optical communication devices.
Secondly, since the conventional light transmitting bodies show a smaller refractive index than the index of a light transmitting body having an ideal refractive index distribution as they depart from the central portion, light which has passed through the peripheral portion of the light transmitting bodies is converged more inwardly (viewed from the direction in which the light advances) on the central axis than the light which has passed through a relatively central portion of the light transmitting bodies. It is noted that since light transmitting bodies are generally in the form of a very fine filament, formation of a convex shape is the only available technique for compensating for aberration, i.e. a deviation in the position of a focal point on the central axis by processing the end face of the light transmitting bodies. However, when the conventional light transmitting bodies having a smaller refractive index than the refractive index of the light transmitting body having an ideal refractive index distribution as they depart from the central portion has an increased aberration when processed into a convex shape.